*{
    margin: 0;
    padding: 0;
    list-style: none;
}
html,body,#root,.box{
 width: 100%;
 height: 100%;
}

header{
    width: 100%;
    height: 40px;
    background: skyblue;
}
main{
    width: 100%;
    height: calc(100% - 80px);
    background: pink;
}
footer{
    width: 100%;
    height: 40px;
    background: skyblue;
}